This invention relates in general to cryogenic cooling systems and more particularly to a miniature dual-split cryogenic system with the compressor section separated from the cooling section.
Miniature cryogenic cooling systems are known and widely used to cool crystals used as radiation detectors. Cooling to cryogenic temperatures reduces the crystal lattice vibrations so as to improve the signal to noise ratio. A particularly important application for such miniature cryogenic coolers is in the cooling of infrared detectors for use in night vision or heat seeking devices. These systems are also useful for medical applications where it is desired to destroy tissue by means of freezing. Although the requirements for such miniature cryogenic systems will vary depending on the end use of the cooler, typical design considerations are their operating efficiency, durability, compactness, weight, microphonics characteristics (generally vibrations resulting from the compressor motor, vibrations within the working fluid, or the physical impact of moving components in the system), and thermophonics (which is noise resulting from thermogradients within the system). For applications involving infrared sensors for airborne devices, all of these design considerations are important.
Known cryogenic cooling systems fall into several categories. In one type the compressor and expander units form an integral system operating on the stirling cycle. Because of the proximity of the compressor and expander sections, such integral systems are particularly susceptible to the effects of microphonics or mechanical vibrations. The systems also tend to be relatively heavy and have typical operating lives of only 300-500 hours. Thus, integral systems are not particularly effective especially for use in airborne systems.
As a way of isolating the compressor vibration from the expander (known as the "cold finger"), split stirling devices are known which separate the compressor system from the cold finger by conduits carrying the working fluid. A typical design problem with such split systems is the acoustic noise generated by an oscillating displacer which is continually being accelerated and decelerated as it oscillates within the cold finger. Such noise is particularly troublesome in single split stirling systems in which but a single conduit extends from a single compressor to the expander.
Dual-split systems have also been developed in an effort to overcome problems in controlling the motion of the displacer so as to minimize microphonics. In such systems two conduits extend from two separate compressors to the expander. In general the pressure waves in each of the conduits are out of phase with one another so that a push-pull arrangement is effected. That is, in such a system the displacer is moved by alternately introducing high pressure on one side and a lower pressure on the other so as to reciprocate the displacer element. One such dual split cooler is disclosed by Durenec in U.S. Pat. No. 4,092,833. The Durenec system employs a compressor with pistons opposed at a 180 degree angle. In this system, however, the compressor pistons are of different areas so that the pressure waves in the individual conduits are of different magnitudes. In addition, there is no suggestion in the Durenec patent of using the split phase relationship to decelerate and reverse the direction of motion of the displacer in a way to prevent contact of the displacer with the walls of the housing. Such contact with the housing or "slamming" not only causes additional microphonics but also audible acoustic noise. Furthermore this Durenec reference neither teaches nor suggests using the cooling effects of a second working volume to precool the working fluid destined for the main working volume at the tip of the cold finger. In addition, the known split phase coolers have short operating lives.
Other known coolers are described in U.S. Pat. Nos. 3,851,173; 4,090,859; 4,078,389; 3,523,427, and 4,206,609.
In particular, U.S. Pat. No. 3,851,173 discloses an integral sterling cycle refrigerator employing a single piston. The compressor section of this refrigerator is known as a "common module" and is in wide use throughout the armed services. Although this refrigerator is an integral sterling cycle device having only a single piston, an important aspect of this invention is the modification of this common module compressor to utilize it with the dual-split cold finger also disclosed herein. In this way, a dual-split sterling cycle cryocooler can be built using many common parts and with a minimum of retrofitting.
It is therefore a principal object of this invention to provide a miniature cryogenic system which is highly efficient, compact and characterized by low level of microphonics and thermophonics.
Another principal object is to provide such a system operating on a dual split "compound" stirling cycle which has a comparatively long operating life.
A further object of the invention is to provide a cryogenic cooler employing a compressor assembly with two isolated compressors and producing a split-phase relationship of 180 degrees.
A still further object of this invention is a cryocooler using a comrpessor whose pistons are oriented at a right angle to one another, namely the "common module" compressor.
A still further object of the invention is to provide a system having all of the foregoing advantages and which is manufactured of conventional materials in a relatively simple and straightforward way.